To catch bottom-dwelling fish, a conically shaped net, called a trawl is dragged along the sea bottom. The trawl is towed by two cables, port and starboard, which are connected to trawl doors which in turn are tied to opposite sides of the mouth of the net. The doors ride over the ocean floor about 100 feet apart and open the net over a wide area.
At the fishing areas, the sea bottom can vary from flat soft mud, to hard sand and gravel with sculpted wave patterns, to rocky cobbles, to outcropping ledge. Often the pattern from soft mud to ledge follows these grades in the order stated. A fisherman would prefer that the net remain over the softer bottom. If the net becomes "hung," i.e. caught upon a rocky hill or ledge, it can cause damage to the fishing gear and loss of time, fuel, and catch. Fish sometimes congregate near rocky outcrops, during fishermen to fish close to the hard (rocky) bottom.
In the old days, a crew member would keep a hand or foot touching the cable at the winch and alert the captain if the cable yanked hard, an indication that the trawl door had struck hard bottom. The captain would then steer away from the side which pulled.
In order to assist the fisherman's efforts to avoid dragging the fishing gear over rocky bottom, today's fishing vessels may be equipped with a depth sounder that is directed downward and/or with a side scan sonar, a device which scans a conic pattern from side to side ahead of the boat. These sensors are not always sufficient to keep the trawl doors from being hung since the net and doors spread to about one hundred feet wide and can lag one to three minutes behind the current vessel position, depending on the depth of water, scope of cable, and speed of forward motion. Wind, waves, and tidal effects keep the net from following precisely behind the fishing vessel.
A fisherman also prefers that the trawl doors "tend the bottom," i.e. that the doors are dragged smoothly along the bottom, holding the net in proper position. Even when the doors are on smooth, soft bottom, the doors can become hydrodynamically unstable and periodically lift from the bottom due to the forces on the door and the cable attachments. This lifting reduces efficiency since the shape of the net changes as the doors rise and fall.